Body fluid is a complex mixture of different cell types and biological particles. Blood, for example, includes plasma and cells (red blood cells, white blood cells, platelets) and the cells occupy about 55% of blood. Plasma is mostly water and it transfers proteins, ions, vitamins, enzymes, hormone, and other chemicals to cells in the body. Red blood cells are about 6 to 8 μm in size and serve to provide oxygen to cells. White blood cells are about 10 to 13 μm in diameter and they defend the body from disease as a part of an immune system by fighting against foreign virus and bacteria. Platelets are the smallest cells, 1.5 to 3 μm, and they stop bleeding by forming blood clots. Fluids in addition to blood, such as saliva, tear, urine, cerebral spinal fluid as well as other body fluids in contact with various organs (e.g. lung) contain mixtures of cells and bioparticles.
The type and amount of cells and bioparticles that are present in a particular body fluid (e.g. blood) includes information about the health of the organism, and in the case of an infected individual, information about the diagnosis and prognosis of the disease. For example, anemia can be diagnosed by counting the number of red blood cells within a unit volume of blood. Similarly, elevated white blood cell count is a standard screen for signs of heightened immune response, which is often due to infection.
In diseases such as HIV, the level of CD4+ T-lymphocytes (CD4+ T-cells) in blood indicates the degree of disease progression. In fact, the CDC Public Health Service recommends monitoring the level of CD4+ T-cells every 3-6 months in all HIV-infected persons as a way to initiate appropriate treatment strategies. Another example is malaria diagnosis, in which the number of parasitized erythrocytes among normal erythrocytes and leucocytes is counted. Yet another example is in cancer diagnosis and prognosis—tumor cells can exfoliate from solid tumors and transport throughout the body via the blood stream or other body fluids (e.g. lung cancer cells may exfoliate into the fluid in contact with the lung and prostate cancer cells into urine). These circulating tumor cells are present in extremely low concentrations, and their isolation and detection among the other cells present in the fluid is required for diagnosis and prognosis.